The present invention relates to thin film batteries. More specifically, the invention is directed toward a flexible thin film battery that has porous electrodes and electrolyte between film layers. The preferred method of mass producing such a battery includes depositing aqueous and non-aqueous solvent inks or coatings in a pattern on a polymeric film layer.
Numerous methods of depositing materials on a substrate are known in the art, including variations of evaporation, chemical vapor deposition, physical vapor deposition, microwave plasma enhanced chemical vapor deposition, sputtering, spray coating, plasma spraying, or vacuum depositing metals. Printing with conductive inks onto polymeric film to make thin film batteries has been tried in the past with varying success. Manganese dioxide and zinc electrodes have been printed from aqueous solutions coating an entire sheet of film. The method of coating or printing in a pattern with a non-aqueous solvent ink or a coating with water swellable polymers on a film layer, such as a polyolefin, is not known to be disclosed as a method of making a thin film battery. A method of using multi-unit printing techniques to form components of a thin film battery in conjunction with converting equipment (besides printing, coating, and laminating) allows other operations to be performed in-line to form a flexible thin film battery or a device incorporating such as a significant component.
Thin, flat batteries have been known in the literature for more than twenty years, but we know of no successful commercial applications. Prior art flat batteries were relatively thick and inflexible. The usable life of such batteries has been too short for many applications. Also, the shelf life of thin film batteries tends to be shorter than other batteries. Also, physical and electrical contact of the cell components in a flat thin film battery has been a limitation in design, which can be overcome by confining the components and sealing film layers around them or by adding binders or adhesives to the components.
It has therefore been found beneficial to produce a flexible thin film battery that is versatile and inexpensive to mass produce. Printed disposable thin film batteries are well suited for low power and high volume applications because they offer adequate voltage, sufficient life, and low-cost solutions.
The invention may be described as a flexible thin film battery. The thin film battery includes a lower film layer, such as polyethylene, that extends beyond the internal components. A portion of the inner surface of the lower film layer preferably has a current collector, such as carbon, printed or coated on a portion of the film. Printed on the cathode current collector is a solvent-based cathode ink that preferably includes manganese dioxide (MnO2), carbon, and a water-swellable polymer. Printed over or inserted on at least a portion of an electrode is an electrolyte-separator layer that preferably includes zinc chloride (ZnCl2) and ammonium chloride (NH4Cl) in a water-soluble film former. The electrolyte-separator layer is disposed to ensure complete physical and ionic contact with the anode and the cathode in the assembled battery.
An anode ink/coating may comprise an organic solvent carrier/binder with zinc particles (such as flakes) and particles of a water swellable polymer. The water swellable particles of the preferred anode expand and permit a greater surface area contact with the zinc in the coating. Expansion occurs when the swellable particles come into contact with liquid associated with the electrolyte. This greater contact area will prolong the life of the cell. The increased life of the battery is obtained using porous anodes and cathodes that have the maximum surface area. Porosity is significant in adding life to a thin film battery, whether it is a dry construction or a wet construction with a gelled electrolyte. Thin film coatings tend not to be porous, but porosity in the thinly applied electrodes is necessary for optimal functioning of the battery. A water swellable polymer can be used with a non-aqueous (organic) solvent-based electrode ink or coating to assist in achieving porosity. The water swellable polymer being a porosity enhancer provides a better life curve for the thin film battery.
The upper layer is a polymeric film, which has an edge that extends beyond the internal battery cell components. The upper layer is sealed around at least a portion of its edges to the lower film layer thereby confining the internal components. xe2x80x9cUpperxe2x80x9d and xe2x80x9clowerxe2x80x9d used in describing the film layers are not a necessity for proper function, but rather for ease of description.
The flexible thin film battery is a breakthrough in the usable life and the shelf life of such a battery. The flexible thin film battery can supply sufficient power for use with numerous devices, and it is versatile when connected in series (greater voltage) or parallel (greater current) to achieve certain desired characteristics. Higher voltage and longer life can be achieved by connecting thin film batteries in series or parallel. Batteries may be printed in mass on a flexible film substrate so that certain numbers and configurations of batteries or devices requiring batteries can be constructed. Interconnections and circuits can also be printed.
The preferred dehydrated state of the battery will be described as a xe2x80x9cdry construction,xe2x80x9d which will be more fully described below as an option. The dry construction represents an electrolyte that is not in a liquid state that is subsequently activated when liquid comes into contact with the electrolyte. An electrolyte can be combined with a porous membrane, water soluble film, or other substances that function as a separator. An electrolyte composition can be printed and dried on a separator, which can be a non-woven superabsorbant fabric. This dehydrated state allows for an extended shelf life. The application of a liquid to a battery having the optional dehydrated state activates the battery.
An alternate battery using this xe2x80x9cdry constructionxe2x80x9d includes an encapsulated liquid that can be released by breaking the encapsulating receptacle. The released liquid activates the battery.
The preferred dry construction has the benefit of providing extended shelf life prior to the application of a liquid while providing maximum power output after liquid is applied.
The flexible thin film battery is environmentally friendly. The construction does not require the use of harmful components, such as mercury or cadmium. The level of acidity or alkalinity is near neutral, so the pH remains near seven. Contact with human skin will not cause injury to that person. The expended flexible thin film battery can be disposed in regular waste removal procedures.
The devices for which this technology can be used are extensive. Any device that requires relatively low power or a limited life may function with a thin film battery. The battery can be inexpensively mass produced so that it can be a disposable product. The low cost allows applications that previously were not cost effective. An example of such a device is a diaper with electronic wetness indicator, for which a concurrent application has been filed.